The present invention relates to a liquid crystal element (for example, a liquid crystal display element or a liquid crystal display) in which a plurality of base bodies each having a liquid crystal orientation film are opposed to each other on the liquid crystal orientation film side with a specific gap put therebetween.
A liquid crystal display (LCD) using liquid crystal as a display element, having a feature allowing a low power consumption with a thin and lightweight structure, is being applied to watches, electronic calculators, computer displays, and television receivers (TVs).
Researches and developments have been actively made to use ferroelectric liquid crystal (FLC) as the above liquid crystal for LCDS. Ferroelectric liquid crystal was first synthesized by R. B. Meyer in 1975, and a surface-stabilized ferroelectric liquid crystal enabling domain inversion by an applied electric field was invented by N. A. Clark and S. T. Lagerwall in 1980. FLC is a liquid crystal whose molecules themselves have permanent dipole moments perpendicular to major axes of the molecules, that is, it has a spontaneous polarization switchable by an applied electric field. A display using FLC has the following excellent features (1) to (3).
(1) The FLC display has a switching speed in the order of .mu.sec, that is, it exhibits a high speed of response being as high as 1,000 times that of a twisted nematic (TN) liquid crystal display.
(2) The FLC display has a molecular arrangement basically containing no twist structure, and thereby it has less dependency on angle of visibility.
(3) The FLC display holds an image after turn-off of a power supply, that is, stores an image, and thereby it allows a simple matrix drive to be adopted for scanning lines of 1,000 lines or more capable of being matched with definition display.
The FLC display is thus expected to meet demands toward high definition, low cost, and large screen.
Such a FLC display (ferroelectric liquid crystal display element) has a structure typically shown in FIGS. 15 and 16. Transparent electrodes 2a and 2b made from ITO (Indium Tin Oxide) of 100 .OMEGA./.quadrature. are provided on transparent substrates 1a and 1b made of glass (Corning Code 7059, 0.7 mm thick), respectively. Each transparent electrode is patterned into a stripe pattern by etching. To be more specific, the transparent electrode 2a are patterned into data electrodes (column electrodes) 2a, and the transparent electrode 2b is patterned into scanning electrodes (row electrodes) 2b. The data electrodes 2a and the scanning electrodes 2b are disposed in such a manner as to cross each other in a matrix.
Liquid crystal orientation films 3a and 3b, represented by oblique vapor-deposition films of SiO, are formed on the transparent electrodes 2a and 2b, respectively. In formation of the oblique vapor-deposition film of SiO, a substrate is disposed obliquely downward from a SiO vapor-deposition source in a vacuum vapor-deposition system. In this case, a deposition angle between a line connecting the vapor-deposition source to the substrate and the normal line of the substrate is set at 85.degree.. The vapor-deposition film of SiO formed on the substrate at a substrate temperature of 170.degree. C. is then baked at 300.degree. C. for 1 hr.
A pair of the substrates 1a and 1b with the orientation films thus prepared are assembled to be opposed to each other on the orientation film side in such a manner that the orientation-treatment direction of the film on the data electrode 2a side is anti-parallel to that of the film on the scanning electrode 2b side, and that the arrangement of the data electrodes 2a is perpendicular to that of the scanning electrodes 2b. As spacers, there are used glass beads 4 having sizes corresponding to a target gap length, for example, glass beads shinshikyu, diameter in a range of 0.8 to 3.0 .mu.m, produced by Catalysts $ Chemicals Industries Co., Ltd.). Although in the example shown in the figures, the orientation-treatment directions of the opposed films are set to be anti-parallel to each other, they may be set to be parallel to each other.
The setting of the spacers 4 is dependent on sizes of the transparent substrates 1a and 1b. In the case where the substrate area is small, a gap between the substrates is adjusted by dispersing about 0.3 wt % of the spacers 4 in a sealing material 6 [UV hardened type adhesive (Photolec, produced by Sekisui Chemical Co., Ltd.) for bonding peripheries of the substrates. In the case where the substrate area is large, the above shinshikyu, are scattered between the substrates at an average density of 100 pieces/mm.sup.2, followed by adjustment of a gap between the substrates, and peripheries of the substrates constituting a cell are bonded by the sealing material 6 with a liquid crystal injection hole being ensured.
As liquid crystal to be injected between the substrates 1a and 1b, there is typically used a liquid crystal composition in which ferroelectric liquid crystal (YS-C152, produced by Chisso Corporation) 5 is homogeneously dispersed at an isotropic phase temperature using a ultrasonic homogenizer. This ferroelectric liquid crystal composition is injected under a reduced pressure at a temperature allowing the liquid crystal to exhibit a fluidity, such as an isotropic phase temperature or a chiral nematic phase temperature. The liquid crystal thus injected is gradually cooled, followed by removal of an unnecessary portion of the liquid crystal adhering on the glass substrates around the injection hole, and the cell is sealed using an epoxy based adhesive, to thus prepare a FLC display 11.
The FLC display 11 is driven by an X-Y matrix system. In the case of using an NTSC system, 1 H (one horizontal scanning time or one selecting time) is set at 63.5 .mu.s, and since a voltage is applied using a bipolar manner in consideration of electrically neutral condition, each selection pulse becomes 63.5/2 .mu.s in width. A select pulse as a threshold value is applied from the row side (electrodes 2b), and a data pulse is applied from the column side (electrodes 2a).
In a ferroelectric liquid crystal element (for example, surface-stabilized ferroelectric liquid crystal element), the orientation of a molecule M is switched between states 1 and 2 shown in FIG. 17 when an electric field E is applied thereto from the exterior. In addition, character Ps indicates a spontaneous polarization. A change in orientation of the molecule M can be converted into a change in transmittance by provision of the liquid crystal element between polarizer sheets disposed perpendicularly to each other. Such a transmittance is rapidly changed depending on the applied electric field, for example, as shown in FIG. 18, it is rapidly changed from 0% to 100% at a threshold voltage V.sub.th. A voltage width in which the transmittance is changed is generally in a range of 1 V or less.
In this way, in the related art ferroelectric liquid crystal display using the bi-stable mode, only the two states are stable, and accordingly, it is difficult to give a stable voltage width to a curve between a transmittance and an applied voltage. In other words, it is difficult or impossible to attain gradation display by voltage control.
The present applicant has studied to solve such an inconvenience and found that an analog gradation display can be achieved by giving a distribution of an effective field strength applied to liquid crystal in one pixel so as to extend a width of threshold voltages for switching between bi-stable states of the liquid crystal in the one pixel, and has already proposed a technique in Japanese Patent Laid-open No. Hei 5-262951 (hereinafter, referred to as "earlier invention").
According to the earlier invention, to achieve the above-described subject "to extend a width of threshold voltages", there is adopted a method of adding and dispersing ultra-fine particles of titanium oxide or the like in ferroelectric liquid crystal.
By addition of the ultra-fine particles in the liquid crystal, micro-domains different in threshold voltage (V.sub.th) appear in one pixel. Since transmittances of the micro-domains are individually changed depending on magnitudes of an applied voltage, the total transmittance of the liquid crystal is changed not rapidly but relatively moderately depending on the magnitudes of the applied voltage, thus enabling analog gradation display. Further, since bi-stable molecules of the liquid crystal have a memory function in one domain and one pixel is formed of these domains (in the order of .mu.m) different in threshold voltage, it is possible to achieve continuous gradation display.
Accordingly, for the liquid crystal element disclosed in the earlier invention, the transmittance is not rapidly changed depending on an applied voltage as shown in FIG. 18 but is relatively moderately changed depending on an applied voltage as shown in FIG. 19. The reason for this is that, as described above, for the liquid crystal disclosed in the earlier invention, since the transmittances of the micro-domains different in threshold value (V.sub.th) appearing in one pixel are individually changed depending on magnitudes of an applied voltage, the total transmittance of the liquid crystal in one pixel is relatively moderately changed depending on the magnitudes of the applied voltage. Further, since bi-stable molecules of the liquid crystal have a memory function in one domain and one pixel is formed of these domains (in the order of .mu.m) different in threshold voltage, it is possible to achieve continuous gradation display.
With respect to the above-described ferroelectric liquid crystal element, however, the present inventor has found that such an element exhibiting the above-described excellent characteristics causes an applied voltage-transmittance hysteresis phenomenon and an after-image phenomenon due to a bipolar moment and a dielectric constant of molecules of the ferroelectric liquid crystal and ions of an impurity in a panel. Here, the hysteresis phenomenon means a phenomenon in which the transmittance is not determined at one value depending on a voltage applied at the previous frame, and the after-image phenomenon means a phenomenon in which a display color selected at the previous frame remains.
In a usual monochromatic ferroelectric liquid crystal display, the above hysteresis and after-image cause problems that if the same image is continuously displayed for a long time, the next image cannot be displayed, and that with the increased power consumption, the drive system is complicated. In particular, in the ferroelectric liquid crystal display of a type containing ultra-fine particles for allowing analog gradation display, in addition to the above disadvantages (hysteresis and after-image), there may occur instability of gradation display color, which makes it difficult to make full use of the advantage of the gradation display of the liquid crystal of this type.
As a result of examination of the above-described hysteresis and after-image, it has been found that one cause of hysteresis and after-image is due to polarization (particularly, electronic polarization) of an orientation film at an interface between liquid crystal and the orientation film generated upon switching of the spontaneous polarization which is a feature of ferroelectric liquid crystal. Such a phenomenon will be described in detail below.
The applied voltage-transmittance hysteresis can be evaluated by measuring a transmittance curve in a condition that an applied voltage is gradually increased and another transmittance curve in a condition in which an applied voltage is gradually decreased, and obtaining a deviation (voltage width) between the two transmittance curves. For example, such a deviation is equivalent to H.DELTA.V shown in FIG. 20. Values of transmittance depending on applied random voltages used for usual image display lie between the two transmittance curves in FIG. 20, and consequently, instability of display color is dependent on a magnitude (H.DELTA.V) of hysteresis.
Accordingly, by eliminating such a voltage width (H.DELTA.V=0), it becomes possible to eliminate an effect of the applied voltage at the previous frame, that is, to select a transmittance at one value only depending on the present magnitude of an applied voltage.
The after-image phenomenon is a phenomenon that in the case of monochromatic display using a usual ferroelectric liquid crystal display, when a voltage (V1 in FIG. 21) for displaying white is applied at a frame next to the previous frame in which black is displayed, black is actually displayed. of course, the reversed phenomenon may occur at the next frame where it is intended to display white. When viewed as the entire display, the image at the previous frame is distortedly seen as a residual image.
This after-image phenomenon appears more significantly in a ferroelectric liquid crystal display of a type containing ultra-fine particles for allowing analog gradation display. To be more specific, since such an after-image phenomenon appears when gray is displayed, as shown in FIG. 22, not only a deviation (H.DELTA.V) is present between the curves but also the curve itself is deformed in a .gamma.-shape. For example, in FIG. 22, although a pattern with a thin color is intended to be displayed in a pattern with a dense color at an applied voltage V2, there is a possibility that a relationship in color between the patterns is reversed. Further, there may occur an unnatural display of a dynamic image. In addition, the after-image can be evaluated by use of a value of Y.DELTA.V as a comparison factor in curves (shown in FIG. 21) which are measured in the same manner as that for obtaining the curve s used to evaluate the hysteresis.
By eliminating these values H.DELTA.V and Y.DELTA.V, it is possible to reduce a value V3 of a data voltage inputted from the column side of a simple matrix and obtain a smooth dynamic image in a usual ferroelectric liquid crystal display, and also it is possible to accomplish a perfect analog gradation display in a ferroelectric liquid crystal display of a type containing ultra-fine particles.